Device to device (D2D) communication provided by the 3rd Generation Partnership Project (3GPP) is a communications technology that allows terminals to use a cell resource to directly communicate with each other in an evolved universal terrestrial radio access network (E-UTRAN).
As shown in FIG. 1, user equipments (UE) not only may communicate with each other by using an evolved Node B (eNB) but also may directly communicate with each other by using an air interface Ud. A Ud interface may provide a direct link (D2D link) between different UEs, and the D2D link may directly transmit, without using the eNB, service data and/or a signaling message between the UEs.
However, a problem is that: when a user equipment (UE) transmits service data and/or a signaling message by using a Ud interface, no corresponding security mechanism can ensure that the transmitted service data and/or signaling message will not be tampered with. Therefore, a security protection mechanism implementing that information is transmitted on a Ud interface is in urgent need.